An improvement on the multi-pin system currently used to control and power
lights and equipment on trailers and semi-trailers and to replace the break-away
cable on smaller trailers
Edited by John Silvester
Project leader
First draft 30 November 2003
Latest update 1st January 2007
Project funded by:
THE BERKSHIRE DRIVING SCHOOL
63 Wheatley
Great Hollands
Bracknell
Berkshire RG12 8UG
Version 26.10.06
This project was initiated by John Silvester, owner of The Berkshire Driving
School. This business trains clients to draw a trailer and pass a driving test
with a trailer. It has been found that the conventional multi-pin trailer plug
connecting the electrics has frequently been unreliable and an improvement was
needed. John Silvester has had success in the past with electro-mechanical
projects. He aims to assemble a team capable of developing an improved trailer
socket which will become the new standard for powering and controlling trailer
lighting and equipment. The present standards do not allow significant
information to be passed back to the towing vehicle and this project is designed
to address this shortcoming.
The present standard for connecting a trailer to a vehicle in order to power electrical devices on the trailer is via a multiple pin connector. This connector typically has seven conductors arranged within a round shell. This shell protects the pins from the ingress of dust and water and prevents the conductors from shorting out should they inadvertently become in contact with the vehicle or trailer chassis. One conducting pin is used for one or a few vehicle lights which illuminate together when voltage is applied to that conductor. The many trailer lamps require many conductors. One conductor is used as an earth and carries the conventional current back to the vehicle. This conductor is typically no larger than the other conductors and this restricts the current carrying capability of the connector. The British system needs a similar connector for charging trailer batteries or running a refrigerator.
The multiple pins and sockets are difficult to clean
Limited current carrying ability for all the pins
The small pins deform when used regularly and contact is lost
The plug is difficult to connect in the dark since it needs to be
correctly orientated. Being round, it is often difficult for the
operator to find the correct orientation
The pins are located mainly by friction and may be easily pulled out
The socket is usually located beside the tow-ball and the cable must
be slack enough to accommodate a bend between trailer and drawing vehicle
The slack cable may touch the ground when the trailer bends the opposite way
Many contacts have to be connected and these are in the plug and socket
which is subject to road dirt and spray thrown up by turbulence at the
back of the towing vehicle
There is no provision for monitoring of equipment on the trailer
A separate break-away cable has to be attached
The current standard has a shutter which has to be lifted.
This means that two hands are needed to effect a connection.
Units are mounted on the towing vehicle and trailer and produces a coded radio signal. This is used to control the lights on the trailer. Signals also pass in the opposite direction and can be used for confirming the code and passing other information from the trailer
A pair of robust connectors carry current to the trailer.
The robust connector should be stout enough to act as a break-away cable
The power cable must be reinforced to act as a break-away cable.
The connector should be placed directly underneath the tow-ball
The connector should locate positively and be unlikely to be disconnected should the tow-ball coupling fail. The connector will then act as the break-away cable attachment.
The standard should allow for future safety devices to be included in the specification.
Trailers towed by cars and leisure 4WD leisure vehicles in Europe use 50 mm
diameter tow-balls.
The absolute maximum towing weight for such vehicles is 1.75 tonnes.
Non-European countries may have higher permitted loads.
Car manufacturers designed towing equipment are often equipped with swan-neck
attachments.
After market and leisure 4X4 vehicles usually use tow-balls mounted on a
tow-bar or chassis plate.
The standard dimensions are:
Mounting hole centres 190mm apart
15mm diameter
Bolt head socket diameter 24mm
End radius 250mm centred around bolt holes
Allow 16mm diameter for socket clearance. This clearance may
be 38mm deep before it would interfere with the torque wrench.
A mild steel mounting plate for electrical connections is
usually sandwiched between the bar and ball.
Ideally the proposed connector would be cast as part of the tow-ball. Due
to the complexities of gaining certification for such a device it is proposed
that the initial design should fit between bar and ball. It will be born in
mind that the design should eventually be incorporated into the tow-ball casting.
Mounting the connector below the ball will reduce the amount of slack
needed to accommodate the vehicle turning.
This position will also offer some protection should the driver not
completely reverse accurately when re-coupling.
The downside of this arrangement is that the operator will not be able
to see the position of the connector below the tow-ball.
This need not be a problem if the connectors lock together easily and
a guiding device is part of the design.
Exposed contacts should not become completely live until hidden during coupling.
Only two pairs of contacts are needed, live and earth.
They will be at a low voltage and low current capability until the trailer
transmits a signal back to the towing vehicle.
When the contacts touch, current flowing to the trailer starts the hand-shaking
process and the towing vehicle allows the full battery voltage and current capability
to be provided to the trailer.
Disconnecting the socket shuts of the main power to the connector.
The rear of a motor vehicle is an inhospitable environment. The two contacts
needed for this project will be less susceptible than the current multi-pin
arrangement but some environmental protection should still be incorporated
into the design.
The socket should be able to resist the maximum trailer weight,
say 1.75 tonnes.
Connecting and removal should be a one-handed process.
Type 631 stainless steel has high strength
Super dolce steel by the Sumitomo (SEI) Steel Wire corporation is
as strong and more corrosion resistant
1x19 stainless steel wire 4mm diameter will have a breaking strength
of about 1.5 tonnes
Sta-Lok structural cable systems can be used to connect to the wire.
Wire and fitting standards manufactured to the forma B.S. 1290: 1983
now to the new standards: bs. 302.pt2: en 12385. bs en13414-1
B.S. 302: Part 1 1987
B.S. 302: Part 2 1987
All corresponding Test certificates supplied
manufactured to the forma B.S. 1290: 1983
now to the new standards: bs. 302.pt2: en 12385. bs en13414-1
B.S. 302: Part 1 1987
B.S. 302: Part 2 1987
All corresponding Test certificates supplied
Trailer Number Plates
The ability for a numberplate to be read is covered by Statutory.
Instrument 1971 No 450 - The Road Vehicles (Registration & Licensing)
Regulations 1971. Part Ill - Exhibition of Licences and Registration
Marks states the following:
Requirements:
The provisions of this regulation shall apply to vehicles,
other than works trucks and agricultural machines, first
registered on or after 1 October 1938.
The registration mark of the vehicle shall be fixed and displayed
on both the front and the back of the vehicle, so that in normal
daylight the letters and figures are easily legible from every
part of the 'relevant area', the diagonal of the square governing
the 'relevant area' being 75 feet.
For the purposes of this regulation, the expression 'relevant area'
in relation to a registration mark on a vehicle means the area
contained in a square described on the ground, either in front of
or behind the vehicle, where one corner of the square is below the
middle of the registration mark and the diagonal of the square from
that corner is parallel to the longitudinal axis of the vehicle, but
excluding any part of the square within 10 feet of the vehicle - see
diagram.
It is an offence to allow the numberplate to be obscured. This is now a
fixed penalty offence, and therefore more likely to be policed. Obviously,
this is of some concern to towbar fitters and to drivers of vehicles with
towbar attachments.
Manchester encoding timed by the rise and fall of the signal
Max bit rate 2,400 bps
ASCII, divide bps by 10 to get cps (characters per second) because
1 start and 8 data and 1 stop bits ASCII will be 240 bytes (or
characters) per second
Half duplex more than halved the data rate since each unit must wait
before receiving before transmitting so signals don´t collide
and Tx must stabilise
Therefore 50 bytes per second in reality (20ms)
Full duplex speeds this up
Possible use of XBee 2.4 GHz modules
History
ZigBee is the name of a wireless communication standard.
It is a development of the virtually defunct Home RF (replaced by
Wireless fidelity or Wi-Fi)
The standard is supported by major players and they have formed a
consortium called the Zigbee Alliance.
XBee is the trade name of MaxStream. These units are new to the
market at the start of this project.
The units are half duplex ie they alternately transmit and receive.
Advantages
The modules are Bi-directional (most modules only transmit in one direction)
It has 16 separate channels so 16 networks can be addressed.
Current consumption is very low so heat build up will not be a problem.
They have transmitting power which can be set within the legal European
standards (10mW)
The units also are low cost.
Transmission speed is quite low but this is not a problem in this application.
Each unit has a unique addressing system whereby the units have unique
factory configured serial number.
The modules have in-built data-packet and error-checking to ensure reliable
data transmission.
They can be easily be interfaced with computers and microcontrollers.
A PICAXE-XBee module is also available
Specification of the XBee module
Range: in the order of 30 m
Transmit power: 1mW
Data rate: 250kbps
Receiver sensitivity: -92dBm
Supply voltage: 2.8-3.4V
Typical transmit current: 45mA
Typical receive current: 50mA
Power down current: less than 10 micro amps
Frequency: 2.4gHz
Dimensions: 244mm by 276mm
Operating temperature: -40 to 85C
Number of pins: 20
Number of channels: 16 direct sequence channels, software selectable
Agency approvals: FCC Part 15.247 Industry Canada (IC) Europe
Figure 1—This is a simplified view of the actual XBee hardware shown in Photos 1 and 2. As long as you don´t overrun the buffers, ZigBee things work as designed.
Please note all information on this site is as correct as best endeavours
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Website created on Sunday 30th November 2003 and last edited on Sunday 13th July 2008.
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